1. Field of the Invention
The present invention relates to a low intermediate frequency receiver, a compensation device, a compensation module and a compensation parameter calculating module, more particularly to a receiver, a compensation device, a compensation module and a compensation parameter calculating module for compensating I/Q mismatch.
2. Description of the Related Art
I/Q mismatch includes characteristics of gain mismatch and phase imbalance. Referring to FIG. 1 and FIG. 2, a waveform plot of a radio frequency (RF) signal, which is received by a conventional low intermediate frequency (IF) receiver, includes a target signal 91 and an interference signal 92. The RF signal shown in FIG. 1 is down-converted from a RF band into a low IF signal 90 shown in FIG. 2, wherein, the low IF signal 90 includes a positive frequency low IF signal having a frequency of fIF, and a negative frequency low IF signal having a frequency of −fIF. Owing to influence resulting from I/Q mismatch, the target signal 91 may mix with the interference signal 92 during down-conversion processing such that the target signal 91 is vulnerable to interference.
In a conventional technology, after a RF mixer down-converts the RF signal, a polyphase filter is usually used for lowering power of the negative frequency low IF signal to be substantially equal to power of the positive frequency low IF signal, and providing signals filtered by the polyphase filter to analog-to-digital converters (ADC) in two I/Q paths. In this way, bit numbers required for performing subsequent analog-to-digital conversion may be reduced.
For solving a problem that the target signal 91 is interfered resulting from I/Q mismatch effect, a prior art for compensating I/Q mismatch effect is disclosed in “Adaptive self-calibrating image rejection receiver” published in International Conference on Communications (ICC) by Ediz Cetin et al., 2004. However, this prior art compensates I/Q mismatch effect by means of a digital image rejection processor (DIRP), and therefore the positive frequency low IF signal and the negative frequency low IF signal resulting from down-conversion processing of the RF signal must have symmetric characteristics. In other words, a scheme of the prior art is only suitable for a receiver including a low-pass filter (LPF) but not a receiver including a polyphase filter.
Another prior art is disclosed in “I/Q Mismatch Compensation Using Adaptive Decorrelation in a Low-IF Receiver in 90-nm CMOS Process” published in IEEE journal of Solid-State Circuits (JSSC) by Imtinan Elahi et al., 2006. This prior art achieves an object of compensating I/Q mismatch effect by means of a complex multiplier. However, similar to the prior art disclosed by Ediz Cetin et al., the positive frequency low IF signal and the negative frequency low IF signal must have the symmetric characteristics. Thus, a scheme of this prior art may not be applicable to the receiver including the polyphase filter.
A compensating technology for I/Q mismatch effect applicable to a polyphase filter scheme is disclosed in U.S. Pat. No. 6,892,060. However, this technology compensates I/Q mismatch effect in an analog manner after down-conversion, and therefore an analog multiplier is required. Nevertheless, accuracy of the analog multiplier has inferior performance compared with that of a digital multiplier, and controllability thereof is not as convenient as utilizing a digital multiplier, such that a higher design cost of the low IF receiver is incurred.